What next, I feel hopeless.

Hey, my ex broke up with me after 3 years, we never had a fight, we lived in an apartment together, we had pets together, we’d discussed having children and buying a house together. One day I came home after work and she just broke up with me with no real reason apart from we had drifted apart. I suggested we discussed it be she refused. I accepted this, moved out and gave her space for a couple of months. After month two I reached out and we had a nice talk via text but since whenever I’ve spoken to her she hasn’t replied quite as much as she did initially.
I found out from one of her friends that she immediately got into a new relationship with her friend that she went to college with and that they had apparently kissed before we broke up. She had been hanging out with him quite a lot towards the end of our relationship but I’m secure enough to have been ok with my partner hanging out with a friend of the opposite sex. The whole situations seems very rebound based/like monkey branching.

It’s been a few months since the break up now and although initially I wasn’t effected by it in this instance time has made things worse for me.
Would anybody have any advice they would like to share on what I could do to try fix things? I’m aware that what she did was wrong but I do feel like if we were able to resolve things we’d both be happy and most probably would go on to get married.

Thank you for any help, suggestions or wise words to could give.